Feher's Patent QPSK (“FQPSK”) is a spectrally efficient form of offset QPSK modulation which uses pulse shaping in order to reduce spectral sidelobes, and cross-correlation between its in-phase (“I”) and quadrature phase (“Q”) baseband signals, in order to maintain a nearly constant signal envelope. These characteristics of FQPSK may make this format desirable for communication [s] in nonlinear, bandwidth-constrained channels.
A special form of FQPSK described in U.S. Pat. Nos. 4,567,602 and 5,491,457 , is known as FQPSK-B. This is a baseband filtered version of FQPSK which is more spectrally efficient than unfiltered FQPSK and thus is useful in limited bandwidth channels. However, the bandwidth limiting of FQPSK-B comes at the expense of bit error rate degradation caused by the introduction of intersymbol interference due to the baseband filtering. For example, a traditional receiver for FQPSK-B may have a bit error rate of 1.4 dB at 10−3.
FQPSK-B signals may be demodulated using symbol-by-symbol detection. This kind of demodulator may be formed of a detection filter along with a sample and hold circuit.
While this forms a simple circuit, the demodulating circuit does not take sufficient advantage of the “memory” that is inherent in FQPSK-B signals. Because of this memory between signals, Viterbi demodulation may be optimum for this type of modulation. Viterbi modulation may provide better bit error rate performance.
A trellis-coded interpretation of FQPSK is known. The FQPSK signal is generated by transmitting one of 16 different shaped waveforms. The basic waveform shapes are shown in Figure 1. Eight unique waveforms are shown in FIG. 1. Eight other shapes, which are the negatives of those waveforms, are also used. These waveshapes characterize the 16 state trellis that represent the optimum Viterbi receiver for FQPSK (or FQPSK-B). A full-blown system of this type, while feasible, may be too complex for a commercial implementation.
A full-blown system of this type, however, may be too complex for a real implementation.